The man-made sun on earth...

...was first preceded by a moment of darkness. Nuclear weapons were contrived invention, but the phenomena behind them are quite natural. The Sun is our main source of energy--directly or indirectly--and at first its light was thought necessary for radioactivity.

Henri Becquerel (1852-1908)

Henri Becquerel was fascinated by phosphorescent materials--materials which glowed in the dark after exposure to sunlight--including uranium salts. In 1896, he knew of Roentgen's discovery the previous year--the discovery of X-rays--and he wondered if phosphorescent uranium would give off penetrating rays after exposure to light.* Here he describes his eureka moment:

One wraps a Lumière photographic plate with a [Ag] bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative...

The shadow of a small copper cross is visible

One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduce silver salts.

But further experiments led him to doubt the necessity of sunlight and to abandon this hypothesis. Later, he reported:

I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts [of potassium uranyl sulfate], arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images.  March 2, 1896

Becquerel's students, Pierre and Marie Curie, went on to discover two new elements, radium and polonium. Marie Curie coined the term "radioactivity" to describe the elements' natural property of begetting penetrating rays.

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*Becquerel may have been aware of Niepce de Saint-Victor's near discovery of the same phenomenon a generation earlier. link